An amplifier is an electronic device that uses transistors or the like to produce an output signal that is an amplified version of an input signal. Types of amplifiers may be broken into classes based on their output performance. For example, class A amplifiers amplify over an entire input cycle such that the output signal is an exact scaled-up replica of the input without clipping, whereas class B amplifiers only amplify half of the input wave cycle. As compared to class A amplifiers, class B amplifiers generate a large amount of output distortion, but have greater efficiencies than do class A amplifiers. A class AB amplifier uses a complimentary pair of class B amplifiers in a push-pull configuration. In such an arrangement, the complementary amplifiers amplify each of the opposite halves of the input signal, which are then recombined at an output. Class AB amplifiers are more efficient than class A amplifiers and have less distortion than class B amplifiers.
Amplifiers can be designed to increase the signal voltage and/or current of an input signal. In terms of supply voltage, amplifiers can operate from double-sided or balanced supplies, which have positive and negative voltage supplies (+ and − supply rails), also referred to as split supplies. The different methods of supplying power result in many different methods of bias to ensure proper operation of the amplifiers. Good control of bias current is necessary because overall current consumption of amplifier devices is a consideration.